This invention relates to a process for preparing catalysts useful in the production of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly it is directed to the preparation of catalysts suitable for producing maleic anhydride from 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or a mixture thereof.
Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or mixtures thereof with molecular oxygen or an oxygen-containing gas to produce maleic anhydride. Conventional methods of preparing these catalysts involve combining a vanadium compound, a phosphorus compound, and if desired, promoter element compounds in a reducing medium under conditions which will provide vanadium in a valence state below +5 to form catalyst precursors capable of being converted to an oxide. The catalyst oxide precursor is then recovered and calcined to provide active catalytic material.
The use of gaseous HCl as a reducing agent for vanadium is disclosed in U.S. Pat. No. 4,002,650 where the vanadium and phosphorus components are present in an aqueous solution. The use of gaseous HCl as a reducing agent for vanadium is also described in U.S. Pat. Nos. 3,864,280; 4,017,521 and 4,043,943 where the vanadium and phosphorus components are reacted in a saturated organic liquid medium. The organic medium described particularly in U.S. Pat. No. 4,043,943, may include saturated, oxygen-containing halogenated organics and inert diluents including halogenated aromatics and hydrocarbons.
Similar preparational techniques are described in European Patent Appln. No. 3,431 in which the additional step of comminuting the vanadium-phosphorus precursor to a particle size of 500 to 700 microns (0.5 to 0.7 mm) is disclosed.
The use of such reducing agents as disclosed in the art, requires special precautions in the preparation of these catalysts because of the corrosive nature of the materials utilized.
U.S. Pat. No. 4,016,105 describes the preparation of vanadium and phosphorus oxide-containing catalysts utilizing as reducing agents organic acids including oxalic, citric, formic, ascorbic and malic, or aldehydes, including formaldehyde and acetaldehyde, together with a co-reducing secondary alcohol. These reducing agents are added to an aqueous solution containing the vanadium and phosphorus components.
A method for preparing catalysts containing vanadium and phosphorus oxides was described in U.S. Pat. Nos. 4,132,670 and 4,187,235 including forming a vanadium-containing compound dispersion in an organic liquid medium such as alcohols, aldehydes, ketones, ethers or mixtures thereof, heating the dispersion to reduce the vanadium, and thereafter adding phosphoric acid in an organic solvent. The organic medium may contain inert diluents, including chlorinated hydrocarbons. These patents teach that organic compounds satisfactory for use in the preparation of such oxides must not contain olefin double bonds.
U.S. Pat. Nos. 4,149,992 and 4,116,868 disclose the preparation of vanadium and phosphorus oxide-containing catalysts prepared from a pentavalent phosphorus-containing compound in a liquid reaction zone. The liquid reaction zone may include aromatic compounds such as benzyl chloride.
In the prior art, separation and recovery of the catalyst precursor from the reaction solution has provided difficulties. Where HCl gas is used as a reducing agent for a vanadium compound, containment and disposal of the excess, corrosive gas required constitutes a problem in commercial scale-up of the catalyst preparation. Conventionally, the solution containing the precursor must be evaporated down, usually to a catalyst precursor-containing paste which must then be dried, broken up and ground. This too provides difficulties for the commercial scale-up of the process, particularly where the catalyst precursor-containing solution includes flammable organic liquids.